The research proposed in this application is one of the first programs of research specifically designed to investigate differences in visual selective attention between persons with and without mental retardation. Mechanisms of selective attention are assumed to be related to many different aspects of cognitive performance. It is therefore important to direct considerable research efforts to the study of attentional functioning of persons with mental retardation. The focus of this application is on: a) assessing the relative abilities of persons with and without mental retardation to restrict visual attention to a limited area of the visual field; b) examining more closely previously observed differences in the relative abilities of persons with and without mental retardation to actively inhibit processing of and responses to irrelevant stimuli in the visual array; and c) investigating the relationship between group differences in visual selective attention and long term learning. Eleven experiments are propose to be conducted during the five years of requested support. It is expected that the results of the proposed research will provide important insight into the relative difficulties that persons with mental retardation have in processing meaningful information. Because most of the cognitive activities in which older children and adults engage require the selection, storage, retrieval, and manipulation of semantic knowledge, this is a domain of research that relates to almost all intelligent behavior.